1. Field of the Invention
The present invention relates to a process for producing a deposit of an inorganic and amorphous protective coating on an organic polymer substrate.
2. Description of the Background
It is known that organic polymer materials, in particular materials such as polycarbonates, require for some practical high-scale utilizations, a significant improvement of their mechanical surface properties. As an example, in the automobile industry, polycarbonates are advantageously used to replace glass, for example, in the optics of headlights, since these materials are lighter and are, overall, better adapted for producing the complex, elongated or rounded shapes of automobile accessories. However, these polycarbonates have a relatively poor surface resistance which rapidly leads to notable and apparent deterioration, which is highly undesirable.
It, therefore, is necessary to provide on the polycarbonate used, a deposit of a surface coating which is both transparent and stable under ambient temperatures and also resistant to abrasion, shock, wear, and other chemical or mechanical acts that may take place, and in particular, to the effects of humidity or corrosion, or the actions of detergents, for example.
Thin layer deposits on such substrates of polymeric materials, and in particular, these based on polycarbonates of the type mentioned above, are especially adapted to be used with silicon based compounds, such as oxides, nitrides, carbides, or alloys thereof, such as oxynitrides. Indeed, these coatings have optical and mechanical properties which are quite suited to the requirements of proposed applications, combined with an appreciable mechanical resistance. However, a difficulty remains in obtaining, during the application of the deposit constituting the coating on the substrate, a suitable adhesion with the latter, this deposit additionally having to be stable in the presence of thermal shocks which may occur under conditions of ambient environment, and the coating having to be continuous and non-porous. Indeed, because of the low thermal stability of the organic polymer substrate, the deposit of the coating must generally be produced at a low temperature, lower than 150.degree. C. for polycarbonates, above which the surface of the substrate is degraded by glass transition of the polymer constituting the substrate. Under these conditions, conventional techniques for forming thin deposits on a support, for example, by the action of an ionized gas flux or a plasma of an appropriate inorganic and amorphous compound, which require temperatures much higher than 200.degree. C., are not suitable.
The same is true for conventional processes which consist in producing a deposit by means of a silicon type liquid resin, which is deposited by soaking the substrate in a bath, followed by polymerization of the deposit in situ under hot conditions, quite apart from the fact that the thus obtained coating does not have the mechanical and optical properties of one which is produced from silicon compounds.
Applicant's own French patent application No. 87 05669 of Apr. 22, 1987 illustrates that it is conventional to provide a process for the protection of a polymer substrate by means of a coating based on compounds, for example, of the silicon oxynitride type, and more generally, of compounds of silicon and oxygen and/or nitrogen and/or hydrogen, which process consists of exposing the surface of the substrate to gas precursors of these elements in the presence of a plasma of an auxiliary gas, but at a temperature lower than that of the glass transition of the substrate, the process taking place in a reactor where the pressure is very low (0.5 to 10 torr) and in which the gases containing the necessary elements are capable of freeing the elements in the body of the plasma so that they can thereafter be recombined together directly on the substrate to form the desired inorganic deposit. Thus, it is especially proposed in this application to use monosilane (SiH.sub.4) or polysilanes (Si.sub.n H.sub.2n+2), or halosilanes of the formula (SiX.sub.n H.sub.4-n), to constitute the silicon precursors, the precursors of oxygen and/or nitrogen being selected from among O.sub.2, N.sub.2 O, N.sub.2, or NH.sub.3.
To operate such a process, it is suitable to use conventional means, particularly to produce the plasma and to confine the latter as well as to regulate the flows of the various gas fluxes required. However, such a process still requires the use of a plasma to afford the thin deposit.